Quantum Frequency Converter

Prepare a photon's polarization state on the Bloch sphere, then send it through one of two wavelength converters. The naive device absorbs the photon to detect it, then re-emits a classical pulse at the new wavelength — this destroys any superposition the photon carried. The quantum-coherent device instead mixes the photon with a strong pump laser inside a nonlinear crystal, shifting its wavelength through sum/difference-frequency generation without ever measuring it, so the exact same superposition survives at the new colour. Compare the live fidelity and coherence readouts between the two methods.